The present invention relates to the field of Hall effect ion ejection devices and more particularly to the field of plasma thrusters.
In the aerospace field, the use of plasma thrusters is well known for notably maintaining a satellite on a geostationary orbit, for moving a satellite from one orbit to a second orbit, for compensating drag forces on satellites placed on a so-called low orbit, i.e. with an altitude comprised between 200 and 400 km, or for propelling a space craft during an interplanetary mission requiring low thrusts over very long time periods. These plasma thrusters generally have an axisymmetrical shape around a longitudinal axis substantially parallel to an ion ejection direction and include at least one main ionization and acceleration channel, obtained in a refractory material surrounded by two circular cylindrical poles, the annular channel being open at its end, an annular anode extending inside the channel, a cathode extending outside the channel, at the outlet of the latter, generally duplicated with a second redundant anode, and a magnetic circuit for generating a magnetic field in a portion of the annular channel. The magnetic field is usually generated by means of electric coils powered by electric generators connected to solar panels.
Although the theoretical operation of these thrusters is still not perfectly mastered, it is generally recognized that they operate in the following way. Electrons emitted by the cathode head towards the anode from the upstream portion to the downstream portion of the annular channel. A portion of these electrons is trapped in the annular channel by the interpolar magnetic field. Impacts between electrons and gas molecules contribute to ionizing the gas introduced into the annular channel through the anode. The mixture of ions and electrons then forms a self-sustaining ionized plasma. The ions ejected downstream under the effect of the electric field generate a thrust of the engine directed in the upstream direction. The ion jet is electrically neutralized by electrons emitted by the cathode 2.
Such plasma thrusters are for example described in American U.S. Pat. No. 5,359,258 and U.S. Pat. No. 6,281,622. Although these thrusters provide an ion ejection velocity, 5 times higher than the ejection velocity provided by chemical thrusters thereby providing a significant reduction in the weight and bulkiness of spacecraft such as satellites for example, this type of thruster has the drawback of requiring heavy and bulky electric generators, and of being expensive. In order to find a remedy to these drawbacks, plasma thrusters with, for a same thrust, reduced consumption of electric current and therefore a reduced mass of electric generators, reduced mass and bulkiness of the magnetic circuit, increased reliability and reduced production cost have already been devised.
This is the case of French patent application FR 2 842 261, for example, which describes a Hall effect plasma thruster, for which at least one of the arms of the magnetic circuit includes a permanent magnet. Said thruster has a longitudinal axis substantially parallel to a propulsive direction defining an upstream portion and a downstream portion, and includes a main ionization and acceleration annular channel made in a refractory material surrounded by two circular cylindrical magnetic poles, the annular channel being open at its upstream end, a gas-distributing annular anode receiving gas from distribution conduits and provided with passages for letting this gas enter the annular channel, said annular anode being placed inside the channel in a downstream portion of the latter, at least one hollow cathode being positioned outside the annular channel, adjacently to the latter, a magnetic circuit including upstream polar ends for generating a radial magnetic field in an upstream portion of the annular channel between these polar portions, this circuit being formed by a downstream plate, from which a central arm located in the centre of the annular channel, two circular cylindrical poles on either side of the annular channel and peripheral arms located outside the annular channel and adjacent to the latter, spring out upstream parallel to the longitudinal axis. At least one of the arms of the magnetic circuit includes a permanent magnet so that the coils for generating the magnetic field have a reduced number of turns, wound in a special high temperature wire. Thus, the reduction in the number of turns allows a reduction in the losses by the Joule effect causing a reduction in the heating of the thruster, an increase in the reliability of the thruster and a reduction in the production cost, the high temperature special wire being brittle and expensive. However, this type of thrusters remains unsuitable for small size thrusters intended for certain applications such as the propulsion of small satellites for example.
Document US 2005/116652 is also known, which describes a plasma thruster with ion ejection including two concentric ionization and acceleration annular channels, one anode extending inside each channel and one cathode extending outside the channels at the outlet of the latter. Said thruster includes a magnetic circuit consisting of electric coils or annular permanent magnets. Moreover, document US 2005/0247885 describes a Hall effect plasma thruster including an ionization and acceleration annular channel, an anode extending inside the channel, a cathode extending outside the channel at the outlet of the latter and a magnetic circuit for generating a magnetic field in the annular channel. The magnetic circuit consists of permanent magnets, a central annular permanent magnet integral with the inner wall of the magnetic circuit and a peripheral annular permanent magnet which is integral with the outer wall and a so-called shunt magnet extending at the bottom of the annular channel. The plasma thruster moreover includes shunt elements with which the magnetic field may be concentrated in order to generate a mirror magnetic field at the outlet of the annular channel, said mirror magnetic field being relatively symmetrical between the poles of the permanent magnets.
Further, document U.S. Pat. No. 5,763,989 describes a plasma thruster including an ionization and acceleration annular channel, an anode extending inside the channel, a cathode extending outside the channel and a magnetic circuit in order to generate a magnetic field in a portion of the annular channel. The magnetic circuit consists of permanent magnets, a central permanent magnet and a peripheral annular permanent magnet. In order to suppress the magnetic field at the anode, the device includes shielding which locally deforms the field lines in proximity to the anode. All these devices require the use of shielding in order to avoid any breakdown at the anode and are unsuitable for small size thrusters.
One of the objects of the invention is therefore to find a remedy for all these drawbacks by proposing an ion ejection device particularly suitable for making a plasma thruster of simple design, inexpensive and having low bulkiness. For this purpose and according to the invention, a Hall effect ion ejection device is proposed, having a longitudinal axis substantially parallel to an ion ejection direction and including at least one main ionization and acceleration annular channel, the annular channel being open at its end, an anode extending inside the channel, a cathode extending outside the channel, at the outlet of the latter, and a magnetic circuit in order to generate a magnetic field in a portion of the annular channel into which a noble gas is introduced, said circuit comprising at least one annular inner wall, one annular outer wall and a bottom connecting the inner and outer walls and forming the downstream portion of the magnetic field; said device is remarkable in that the magnetic circuit is laid out so as to generate at the outlet of the annular channel a magnetic field independent of azimuth and, in the area of the anode, a magnetic field for which the radial component is zero.
It will be noted that the fact that the magnetic field is independent of azimuth, provides at the outlet of the annular channel a globally constant and quasi-radial magnetic field regardless of the azimuth. In this way, the electrons arriving in the outlet area of the annular channel with a velocity parallel to the axis of revolution of the device are subjected to a Laplace force which imparts a cyclotron movement to them in the outlet plane of the annular channel. The electrons are thus massively trapped in the outlet area causing an increase in the probability of ionizing collisions with the atoms of the noble gas. Further, the radial component of the magnetic field is zero in the area of the anode; the device does not require shielding in order to deform the field lines.
The device includes a so-called central annular permanent magnet integral with the inner wall of the magnetic circuit and a so-called peripheral annular permanent magnet integral with the outer wall of the magnetic circuit and for which the magnetization direction is opposite that of the central magnet. Moreover, the bottom of the annular groove includes an annular through-recess forming a gap. Advantageously, the central and/or peripheral magnet includes a plurality of magnetic elements positioned in a circular way. Further, the central and/or peripheral magnet includes one or more amagnetic elements. Each magnetic element of the peripheral magnet has a determined power. Said elements of the central and/or peripheral magnet are cylinders obtained in a metal SmCo alloy.
According to an alternative embodiment of the device according to the invention, the central and/or peripheral magnet is obtained in hard ferrites, so-called hexaferrites. Advantageously, the magnetic circuit is obtained in soft ferrites which are preferably selected from the following list of ferrites of general formula MFeO4 or MO, Fe2O3.
Moreover, the device includes an annular part obtained in a porous refractory material and positioned in the bottom of the annular groove in order to cap the gap and close the bottom of the annular channel. This annular part is preferably obtained in porous ceramic. Further, the anode has an annular shape and extends in the middle portion of the annular channel. The device will find many industrial applications 1 such as a Hall effect plasma thruster or a device for a surface treatment with ionic implantation for example.